Sideband modulation



Aug. 1, 1939. N E. UNDENBLAD SIDEBAND MODULATION Filed March 19, 1937 2 Sheets-Sheef 1 anu ugl 1939 y N, EILINDENBLAD 2,167,722

SIDEBAND MODULATION Filed March 19, 1937 2 Sheets-Sheet 2 Lowa/uva PASS ,ll/GH A/vo m55 AAAAAA VVVVVVV INVENTOR "'ML/NDENBLAD ATTORNEY Patented Aug. 1, 1939 UNITED STATES airain rarer SIDEBAND MODULATIN tion of Delaware Application March 19,

13 Claims.

This application concerns an improvement in methods of and means for producing single sideband modulation of a carrier wave in accordance with television signals.

The theoretical band width on each side of the carrier required for the transmission of 30 frames of 343 line pictures is considerably above 1.5 megacycles. By taking psychological and physiological factors into consideration it has been found possible to keep the required modulation band on each side of the carrier down to 1.5 megacycles, This represents a band width of 3% on each side of a 50 megacycle carrier.

A 3 to 5 percent band with an impedance or admittance variation corresponding to less than l db. is about the average obtainable with simple radiating structures without additional artificial loading. Thus, it is feasible to use a modulation band width of 3% of the carrier efficiently with radiating systems in present use.

For the sake of efciency it is desirable to use as long a wave as possible. This facilitates generation of power and permits coverage of a greater area successfully with a given power 25 broadcast. However, when one decreases the carrier frequency the percentage band width on each side of the carrier increases for a given modulation frequency band. Thus, the extent to which the carrier frequency can be decreased is also limited in that a radiating structure, unless quite complicated, will not respond uniformly over the entire band. A simple radiating structure will no longer give a constant response throughout the sideband range and precautions, such as artiiicial loading, etc., of the antenna must be taken to prevent variation of the load through the filter circuits on the output circuit. Thus, a compromise must be made which will satisfy both conditions given above insofar as possible. In a known example of modulators of this type a carrier of 50 megacycles is selected and after amplification, is modulated by energy characteristic of 30 frames of 343 line pictures. The modulation output is then impressed on two sideband pass filters tuned to the upper and lower side frequencies respectively. One lilter is terminated in an artificial load, the other in a radiating structure. In this arrangement, the changes in response of the power amplifier tank circuit and of the radiating structure or load for the different frequencies of the wide frequency band covered by one sideband results in variations in load on the filters, the power amplifier, etc., and these variations produce reactions through the power amplifier to the succeeding amplifiers and cause 1937, Serial No. 131,828

(Cl. Z50-47) cross talk, inter-modulation, etc. It might be thought that the filter between a one way system would prevent variations in load from reacting on the power amplifier but this is not entirely true because a lter is only as good as its terminating elements and the filter passing the used sideband terminates at one end in a radiating structure and at the other end is coupled to a tank circuit also coupled to the other filter which terminates in an artificial load. The radiating structure or the artificial load or both, may not respond entirely linearly throughout the wide frequency sideband ranges involved and in this case variations in the lter terminating elements will act through the filter on the power amplifier output and from there to the other filter and on preceding stages. It might also be thought that a power amplifier is a linear device but actually its internal impedance changes with the external impedance. Therefore, if one sideband filter is variably loaded there will be reactions on both sidebands and these in turn may act through the power amplier on the preceding stages.

The object of my present invention is to provide a new and improved sideband modulation system which eliminates the defects enumerated above and by means of which a single sideband modulated by a wide modulating potential band may be produced, amplified, and impressed on appropriate radiating structure.

This invention therefore, in its broadest aspect, concerns a new and improved means for transferring from one tuned circuit in which wave energy covering a relatively wide frequency spectrum flows to a second circuit, wave energy covering a portion only of said spectrum and so loading said first circuit as to improve its impedance characteristic over the entire range of the frequency spectrum. In a limited application the transferred frequency spectrum is characteristic of the entire band.

This is accomplished by transferring the desired frequency spectrum by means of a filter and coupling to the rst tuned circuit a network which imposes on said circuit an impedance characteristic such as to be complementary to the characteristics imposed on said circuit by said iilter transferring the selected band, and providing thereby a substantially constant impedance over the entire band of frequencies in the first named tuned circuit,

In describing my invention reference wili be made to the attached drawings wherein,

Figure 1 illustrates the essential elements of the improved single sideband modulator as described hereinbefore;

Figure la is a modification of a portion of the circuit of Figure 1. In Figure 1 plate modulation is used. In Figure 1a grid modulation is used;

Figure 2 is a curve illustrating the character of the output of the modulator as impressed on the band pass filters and also the character of the output, the power amplifier as impressed on the load or transmission line; while Figures 3, 4, 5 illustrate band pass filters of different character which may be utilized in place of the band pass filters of Figure l.

Referring to the drawings, and particularly to Figure l, l5 is a modulator comprising a pair of electron discharge devices having their control grids connected by reactance I2 which may be coupled for excitation to any source of carrier wave energy of the desired frequency and amplitude. The anodes of the modulator tubes are connected together by a tuned tank circuit I, a` point on which is connected to the anode of a modulator tube l2, the control grid of which is connected to a source of modulating potentials I4 and the anode of which is connected by a reactor I6 to a source of direct current potential. If desired, the anode of each of the carrier wave amplifier tubes in the stage I0 may be connected to ground as shown by external tank loading impedances.

'Ihe modulated output from tank circuit I is impressed on the inputs 20, 22 of a high band pass filter and a low band pass filter respectively. This arrangement permits susbtantially complete separation of the upper and lower sidebands except for possibly the lowest modulation frequencies. If the upper sideband energy is to be utilized the output of the high band pass filter is connected as shown to the control or other input electrode of the initial tube in a power amplifier 24. This power amplifier may be of any type but is preferably of a type such that changes in load in the output thereof cannot react therethrough to affect the high band pass filter. For example, a neutralized power amplifier is appropriate for use here or an amplifier wherein tubes of the screen grid type wherein the screen grid shields the output electrodes from the inputk electrodes. Obviously, amplifiers of the electron coupled type may also be used. By the terni amplifiers one or more tube stages in cascade or stages comprising parallel tubes in cascade, is contemplated. The output of the power amplifier 24 is connected directly to a radiating structure or by lines to any utilization circuit. The low band pass filter is terminated in an artificial load such as, for example, an absorbing resistance 30 wherein the energy representative of the lower sideband frequencies is expended. The gain from this procedure lies in the fact that thanks to the neutralizing in the power amplifier 24 or the un-coupling between the output and input thereof, the impedance from antenna, line 3E, and cross talk filter is uri-coupled from the modulated stage Il). The high pass filter is now clearly terminated for all frequencies in the high frequency sideband including those of the lower sidebands which may inadvertently get through the high pass filter. The high pass filter now acts as a true filter whereas if its output impedance varies filter tuning conditions may arise if the high pass filter lets through portions of the lower sidebands. This condition might occur if the high band pass filter was located between the power amplifier and the antenna or lines 30, as has been the Case heretofore because the antenna impedance is of variable value for the lower sidebands, being adjusted for linear response over the higher sideband frequencies.

The tank circuit of the power amplifier 24 may now be given characteristics such as to amply take care of the upper side frequency when this amplifier is operated class A. While class A operation is not efiicient it gives high fidelity and is reliable. Obviously it would be very difficult to make this stage 24 broad enough to have a flat pass characteristic for both the upper and the lower sidebands.

The power of the modulated stage I El may have to be somewhat greater than normal since stage I0 will have to be loaded not only by some artificial loading of the high pass filter but also by the loading from the low pass filter and absorber resistance and, in general this stage EG, must be broad enough to pass both sidebands well. Thus, the characteristic of the tank circuit I must be such as to pass substantially uniformly all of the frequencies covered by the carrier and both sidebands. Obviously, in order to obtain this characteristic excessive modulation in the stage ID must be avoided.

Moreover, it remains to be understood that excessive modulation must be avoided when using single sideband because of phase modulation components causing distortion at the higher modulation levels.

Since the lower modulation frequencies go all the way down to 2O cycles, it is course necessary to allow both sidebands to pass through the high pass filter at the lower modulation frequencies. This has been indicated by the curve in Figure 2. In this figure the high band pass and low band pass filter characteristics are indicated in full lines. The tuning curve of the tank circuit I, which covers both sidebands, has been indicated by the dotted lines as has the tuning curve of the power amplifier which is in effect, the tuning curve of the tank circuit of the amplifier 24. It is entirely feasible to permit the upper sideband and the higher frequencies of the lower sideband to pass through the high pass filter as long as the carrier is located in the lower slope of the high pass filter as shown in Figure 2. The high and low pass filter characteristics cross one another at the carrier frequency as shown. In this manner, I maintain the total sideband power almost constant throughout the entire modulation band.

Although I have used plate modulation in the preferred embodiment of Figure 1, it will be understood that if desired grid modulation may be used. In this case the modulator may be as illustrated in Figure la. An advantage of the arrangement of Figure la is that due to the well known balanced modulator effect the carrier wave is substantially suppressed from the output or circuit I and this simplifies the filtering and compensating action of the present invention.

While the high band pass filter and the low band pass filters of Figure i have been shown as comprising series and parallel reactances of the capacitive type, it will be understood that I contemplate the use of any appropriate filters in these circuits. For example, the high band pass filter of Figure l may be replaced by the high band pass filter of Figure 3. The low band pass filter of Figure 1 may be replaced by the low band pass filter of Figure 4 and finally the high and low band pass filters of Figure 1 may be replaced by the high or low band pass filter of Figure 5 provided the elements thereof are given the proper values.

I claim:

1. In a transmission system comprising in combination a modulator, a source of carrier wave energy, and a source of modulating energy interconnected to produce modulated wave energy, a load circuit, a filter circuit tuned to the frequency range between the carrier and one maximum sideband frequency of said wave energy connected between the modulator and the load circuit, and a complementary lter terminated by an artificial load tuned to the frequency range between the carrier and the maximum sideband on the opposite side of said carrier relative to said first mentioned maximum sideband frequency connected with the modulator for balancing the modulator for all frequencies.

2. In a single side band signalling system for transferring a portion only of the frequency spectrum of a wide frequency band comprising a carrier and side bands from a tuned circuit to a load circuit, a filter circuit coupling said load circuit to said tuned circuit, said filter circuit boing tuned` to pass a selected band of frequencies less than the total frequency spectrum but including a side band from said tuned circuit to said load circuit, and a network tuned to a selected band of frequencies not passed by said filter substantially coupled to said tuned circuit only, said network imposing on said tuned circuit impedance characteristics complementary to the impedance characteristic imposed on said tuned circuit by said filter circuit.

3. In a system for transmitting a predetermined portion only of a total band of frequencies, produced by modulating a source of carrier wave energy by modulating potentials, to a load circuit by means of a filter circuit for selecting said portion only of the total band of produced frequencies which is connected between the load circuit and the modulator, the combination which comprises a complementary compensating circuit tuned to the remaining portion of the total frequency band and connected to said modulator only for loading said modulator at a portion of the remaining frequencies within the produced band so that said modulator is substantially uniformly loaded for all produced frequencies.

4. In a television system, the combination of a `load circuit, a modulated wave amplifier having an output coupled to said load circuit, said amplifier having an input, means for preventing reaction between the output and input of said amplifier, a carrier wave amplifier having its anode coupled to a tuned tank circuit, its control grid excited by wave energy to be modulated and one of its electrodes coupled to a source of modulating potentials to produce modulated energy comprising sideband frequencies, a filter tuned to pass a substantial amount of said modulated energy coupling said tank circuit to the input of said power amplifier, an artificial load, and a filter tuned to pass a substantial amount of said modulated energy coupling said tank circuit to said artificial load, said filters being tuned to pass wave energy of different frequency.

5. In a television system, the combination of a load circuit, a neutralized amplifier having an output coupled thereto, said neutralized amplifier having an input, a carrier wave amplifier having its anode coupled to a tuned tank circuit, its control grid excited by wave energy to be modulated and one of its electrodes coupled to a source of modulating potentials to produce modulated energy comprising sideband frequencies and impress the same on said tank circuit, a band pass filter coupling said tank circuit to the input of said power amplifier to supply modulated energy to said power amplifier from said tank circuit, and artificial load, and a band pass filter coupling said tank circuit to said artificial load, said .last filter being tuned to pass energy not passed by said first filter to thereby ,load said tank circuit substantially uniformly for a selected band of frequencies.

6. In a television system, the combination of a load circuit, an amplifier having an output coupled thereto, said amplifier having an input, means for preventing said output circuit and load from reacting on the input of said amplifier, a carrier wave amplifier having its anode coupled to a tuned tank circuit, its control grid excited by wave energy to be modulated and its anode electrodes coupled to a source of modulating potentials to produce modulated energy and impress it on said tank circuit, a high pass filter coupling said tank circuit to the input of said power amplifier to supply modulated energy thereto for signalling purposes, and means for substantially uniformly loading said tank circuit over the modulated energy frequency range comprising, an artificial load, and a low pass filter coupling said tank circuit to said artificial load.

'l'. In a system for transferring wave energy modulated in accordance with signal potentials covering a wide frequency band from a circuit tuned to the mean frequency of said wave energy to a load circuit tuned to respond linearly to a single side band only of said modulated wave energy, a power amplifier adapted to repeat voltages in one direction only having an output coupled to said load circuit, said amplifier having an input, a low pass filter coupled at its input to said tuned circuit and at its output to an articial load, and

a high pass lter coupled at its input to said tuned circuit and at its output to the input of said power amplifier, said filters together imposing a .load on said tuned circuit which is substantially uniform over the wide frequency band to improve its characteristic and prevent reaction between diiferent frequencies of the wide band.

8. In means for impressing a wide band of modulating potentials on ultra-high frequency wave energy, an electron discharge device having a control electrode on which said ultra-high frequency wave energy may be impressed, said tube having an output electrode, an output circuit connected to said output electrode, means for impressing modulating potentials which cover a wide frequency band on the impedance of said device to modulate said high frequency wave in accordance with said wide band of modulating potentials, a power amplifier having an output connected to a load, said power amplifier having an input, a filter for passing the upper sideband connecting said output electrode to said power amplifier input and a filter for passing the lower sideband coupling said output electrode to an artificial load, said two filters imposing on said tube output complementary impedance characteristics to thereby improve its linearity of operation over the wide band of frequencies through which said ultra-high frequency wave energy is modulated.

9. In means for impressing a wide band of modulating potentials on ultra-high frequency wave energy, an electron discharge device having a control electrode on which said ultra-high frequency wave energy may be impressed, said device having an output electrode, means for modulating the impedance of said device in accordance with modulating potentials, a tank circuit connected to said output electrode and tuned to the mean frequency of said ultra-high frequency wave energy, an external .load connected with said output electrode to broaden the frequency range to which said tank circuit responds linearly, a power ampliiier having an output connected to a load, said power amplifier having an input, a band pass filter operating over a first band of frequencies connecting said output electrode to said power amplifier input and a band pass iilter operating over a second band of frequencies coupling said output electrode to an artificial load.

l0. In means for impressing a wide band of modulating potentials on ultra-high frequency carrier wave energy, an electron discharge device having a control electrode on which said ultrahigh frequency Wave energy may be impressed, said device having an output electrode, means for impressing modulating potentials on the anode of said device whereby carrier and side bands are produced in said device, a tank circuit connected to said output electrode and tuned to the mean frequency of said Wave energy, an external load connected with said output electrode to broaden the frequency range to which said tank circuit responds linearly, a neutralized or equivalent power amplifier of the class A type having an output connected to a load, said power amplifier having an input, a filter for passing the upper sideband connecting said output electrode to said power amplifier input and a filter for passing the lower sideband coupling said output electrode to an artificial load.

ll. In a modulation system, a pair of electron discharge devices each having a control grid, a cathode, and an anode, means for impressing wave energy in phase opposition on said control electrodes, a tank circuit connecting said lanodes in push-pull relation, a tube having an anode, a cathode, and a control electrode, a connection between the anode of said tube and a point on said tank circuit, a source of modulating potentials connected with the control grid of said tube, a load circuit, an amplifier having an output coupled to said load circuit, said ampliiier having an input, an artificial load circuit, and band pass filters operating over diierent frequency ranges coupling said tank circuit to said amplifier input and to said load circuit respectively.

12. In a modulation system, a pair of electron discharge devices each having a control grid, a cathode, and an anode, means for impressing wave energy in phase opposition on said control electrodes, a tank circuit connecting said anodes in push-pull relation, a tube having an anode, a cathode, and a control electrode, a connection between the anode of said tube and a point on said tank circuit, a source of modulating potentials connected with the control grid of said tube, a load circuit, a neutralized amplifier having an output coupled to said load circuit, said neutralized amplifier having an input, an artificial load circuit, and high and low pass filters coupling said tank circuit to said amplifier input and to said load circuit respectively.

13. In a modulation system, a pair of electron discharge devices each having a control grid, a cathode, and an anode, means for impressing wave energy in phase opposition on said control electrodes, a tank circuit connecting said anodes in push-pull relation, an external impedance connecting the anode of each device tothe cathode of each device to broaden the characteristic of said tank circuit, a tube having an anode, a cathode, and a control electrode, a connection between the anode of said tube and a point on said tank circuit, a source of modulating potentials connected with the control grid of said tube, a load circuit, a neutralized amplifier having an output coupled to said load circuit, said neutralized amplifier having an input, an artiiicialload circuit, and high and loW passvfilters of different character coupling said tank circuit to said amplifier input and to said load circuit respectively.

NILS E. LINDENBLAD. 

